1. Field of the Invention
The present invention relates to a magnetoresistive element using a magnetoresistance effect and a thin film magnetic head comprising the magnetoresistive element.
2. Related Art Statement
Under a tendency of miniaturization of a magnetic disk drive unit, a thin film magnetic head using a magnetoresistive element (hereinafter, referred to as an MR element also) as a read head is known as a magnetic/electric converter suitable for reading information stored in a high recording density on a magnetic medium since its output is not related to the relative speed of the head to a magnetic disk.
As an MR element used in such a thin film magnetic head, an MR element using an anisotropic magnetoresistance effect film (hereinafter, referred to as an AMR film) made of permalloy and the like, and a gigantic magnetoresistive element (hereinafter, referred to as a GMR element) using a spin valve film structure (hereinafter, referred to as an SV film structure) are known. These MR elements each contain a magnetic sensitive film responding to a magnetic field applied from the outside. In case of an AMR element an AMR film corresponds to the magnetic sensitive film, and in case of a GMR element of a spin valve structure a free layer corresponds to the magnetic sensitive film. A longitudinal bias magnetic field (a single domain anisotropic magnetic field) is applied to the magnetic sensitive film by a magnetic domain control film. This longitudinal bias magnetic field makes the magnetic sensitive film into a single magnetic domain and suppresses a Barkhausen noise to be caused by movement of a domain wall. As a method for applying a longitudinal bias magnetic field, an exchange bias method utilizing the exchange coupling between an antiferromagnetic film and a magnetic film, and a hard magnetic bias method using a hard magnetic film are known.
For example, an exchange bias method is disclosed in Japanese Patent Laid-Open Publication No. Sho 62-40,610 and U.S. Pat. No. 4,103,315. The respective magnetic domain control films in an MR element disclosed in these prior technical documents are stacked with a space between them on both sides being opposite to each other of an AMR film to be a magnetic sensitive film. A magnetic domain control film comprises an antiferromagnetic film. And utilizing an exchange coupling generated between an antiferromagnetic film and a magnetic film, this method applies a uniform longitudinal bias magnetic field to the AMR film to make it into a single magnetic domain and thereby prevents a Barkhausen noise to be caused by movement of a magnetic domain.
The specification of U.S. Pat. No. 5,528,440 discloses an exchange bias method which makes a magnetic domain control film have a film structure generating an exchange coupling by forming an antiferromagnetic film of an NiMn layer on an NiFe layer and brings an end portion of this magnetic domain control film into contact with an AMR film.
Japanese Patent Laid-Open Publication No. Hei 7-244,821 discloses a technique which removes an antioxidizing film on the surface of an AMR element layer by cutting by a specified amount both end portions in the longitudinal direction of the AMR element, forms an antiferromagnetic film in an area where an exchange coupling can be generated, generates an exchange coupling between a magnetic film and the antiferromagnetic film, and applies a longitudinal bias magnetic field.
Japanese Patent Laid-Open Publication No. Hei 7-210,834 discloses a film structure having magnetic domain control films provided on both ends in the longitudinal direction of an AMR film, wherein a buffer layer composed of an NiFe film is provided between the AMR film and the magnetic domain control film.
Further, a hard magnetic bias method has been disclosed in Japanese Patent Laid-Open Publication No. Hei 3-125,311. In this prior technical document, hard magnetic films are formed in end passive areas at both sides of it.
A GMR element having an SV film structure is disclosed in Japanese Patent Laid-Open Publication No. Hei 4-358,310 and IEEE TRANSACTIONS ON MAGNETICS, Vol. 30, No. 6, NOVEMBER 1994, pp. 3801-3806. This known SV film structure has basically a structure having a pin layer and a free layer stacked with a nonmagnetic layer between them. The pin layer has the direction of magnetization fixed in one direction, and the free layer has the direction of magnetization freely moving in response to a magnetic field applied from the outside. The SV film structure has the minimum resistance value when the pin layer and the free layer have the same direction of magnetization, and has the maximum resistance value when they are inverse to each other in direction of magnetization. An external magnetic field is detected by utilizing this resistance change characteristic.
In the SV film structure also, a magnetic domain control film for applying a longitudinal bias magnetic field to a free layer is provided. As a magnetic domain control film, a film structure by an exchange bias method and a film structure by a hard magnetic bias method are known.
Although a longitudinal bias magnetic field by a magnetic domain control film is necessary for making a magnetic sensitive film into a single magnetic domain, a too large longitudinal bias magnetic field degrades the reproduction sensitivity of the magnetic sensitive film. Accordingly, the intensity of a longitudinal bias magnetic field needs to be set so as to make a magnetic sensitive film into a single magnetic domain but so as not to degrade the reproduction sensitivity of the magnetic sensitive film. The intensity of a longitudinal bias magnetic field varies according to the relation between the thickness and magnetic characteristics of a magnetic sensitive film and the thickness and magnetic characteristics of a magnetic domain control film. In order to set a proper longitudinal bias magnetic field, therefore, it is necessary to clarify an influence which the thickness and magnetic characteristics of a magnetic sensitive film and the thickness and magnetic characteristics of a magnetic domain control film have upon the longitudinal bias magnetic field.
However, the above-mentioned prior technical documents have disclosed methods for applying a longitudinal bias magnetic field to a magnetic sensitive film and thereby making it into a single magnetic domain, but they do not teach at most the relation between the thickness and magnetic characteristics of a magnetic sensitive film and the thickness and magnetic characteristics of a magnetic domain control film, said relation having an influence upon the longitudinal bias magnetic field.
And in order to cope with high-density integration of an MR element, it is necessary to make thinner the thickness of a magnetic sensitive film. For example, in case of an AMR element, it is required to reduce the thickness of a magnetic sensitive film from a thickness of 20 to 25 nm at present to a thickness of 10 nm or less. The above-mentioned prior technical documents do not teach how to determine the thickness of a magnetic domain control film in case that the magnetic sensitive film is made thin as described above.
An object of the present invention is to provide a magnetoresistive element capable of applying a proper longitudinal bias magnetic field to a magnetic sensitive film according to a change in thickness of the magnetic sensitive film, and a thin film magnetic head provided with the same magnetoresistive element.
Another object of the present invention is to provide a magnetoresistive element capable of applying a proper longitudinal bias magnetic field to a magnetic sensitive film by clarifying an influence which the magnetical thickness of a magnetic sensitive film and the magnetical thickness of a magnetic domain control film have upon the longitudinal bias magnetic field, and a thin film magnetic head provided with the same magnetoresistive element.
A further other object of the present invention is to provide a magnetoresistive element capable of applying a proper longitudinal bias magnetic field to a magnetic sensitive film even in case of making thin the magnetic sensitive film, and a thin film magnetic head provided with the same magnetoresistive element.
A magnetoresistive element according to the present invention comprises a magnetic sensitive portion and a magnetic domain control film. Said magnetic sensitive portion comprises a magnetic sensitive film, and said magnetic sensitive film is a film responding to a magnetic field applied from the outside. Said magnetic domain control film is a film applying a longitudinal bias magnetic field to said magnetic sensitive film in case of assuming that the direction in which a sense current flows is the longitudinal direction. In case of assuming that a magnetical thickness given by the product of the remnant magnetization Br1 (gauss) of said magnetic sensitive film and its film thickness THx (xcexcm) is x (gauss-xcexcm), that a magnetical thickness given by the product of the film thickness THy (xcexcm) of said magnetic domain control film and its remnant magnetization Br2 (gauss) is y (gauss-xcexcm), and that the minimum magnetical thickness of said magnetic domain control film determined in consideration of the minimum magnetic field Hmin required for operation, of a longitudinal bias magnetic field to be applied to said magnetic sensitive film is xcex1 (gauss-xcexcm), the present invention satisfies the following expression:
y greater than =xe2x88x922x+xcex1.xe2x80x83xe2x80x83(1)
Said magnetic domain control film makes a magnetic sensitive film into a single magnetic domain by applying a bias magnetic field in the longitudinal direction in which a sense current flows to said magnetic sensitive film, and as a result it can suppress a Barkhausen noise.
Next, the meaning of the above-mentioned expression (1) xe2x80x9cy greater than =xe2x88x922x+xcex1xe2x80x9d is described. According to studies of the present inventors, a longitudinal bias magnetic field (single axis anisotropic magnetic field) to be applied to a magnetic sensitive film varies linearly in proportion to the magnetical thickness x of a magnetic sensitive film. This means that as the magnetic sensitive film becomes thinner, the single axis anisotropic magnetic field (longitudinal bias magnetic field) becomes smaller and an action of suppressing a Barkhausen noise is degraded, and that there is the minimum longitudinal bias magnetic field Hmin capable of suppressing the Barkhausen noise. Here, it is assumed that the ratio of a unit change quantity xcex94H1 of a longitudinal bias magnetic field to a change xcex94x of a magnetical thickness corresponding to the unit change quantity xcex94H1 is xe2x80x9cxcex94H1/xcex94xxe2x80x9d.
In the meantime, a longitudinal bias magnetic field generated by a magnetic domain control film varies also linearly in proportion to its magnetical thickness y. Namely, as the magnetical thickness y becomes larger, the longitudinal bias magnetic field generated by the magnetic domain control film becomes larger. Here, it is assumed that the ratio of a unit change quantity xcex94H1 of a longitudinal bias magnetic field to a change xcex94y of a magnetical thickness corresponding to the unit change quantity xcex94H1 is xe2x80x9cxcex94H1/xcex94yxe2x80x9d.
Paying attention to the above-mentioned characteristic, the present invention increases the magnetical thickness y of a magnetic domain control film and increases a longitudinal bias magnetic field generated in the magnetic domain control film when the magnetical thickness x of a magnetic sensitive film becomes smaller and its longitudinal bias magnetic field is lowered. By this, the invention compensates for lowering of a longitudinal bias magnetic field caused by decrease of the magnetical thickness x of a magnetic sensitive film. Since magnetical thickness x and y each are the product of a film thickness and a remnant magnetization, the magnetical thickness x and y each can be adjusted by selecting the film thickness. Therefore, in case that the film thickness of a magnetic sensitive film has varied, it is possible to apply a proper longitudinal bias magnetic field by increasing the film thickness of a magnetic domain control film in accordance with the change in thickness of the magnetic sensitive film.
Next, according to studies of the present inventors, it has been known that the ratio xe2x80x9cxcex94H1/xcex94xxe2x80x9d of a unit change quantity xcex94H1 of a longitudinal bias magnetic field to a change xcex94x of a magnetical thickness corresponding to the unit change quantity xcex94H1 is about double the ratio xe2x80x9cxcex94H1/xcex94yxe2x80x9d of a unit change quantity xcex94H1 of a longitudinal bias magnetic field to a change xcex94y of the magnetical thickness corresponding to the unit change quantity xcex94H1.
Therefore, in case of compensating for lowering of a longitudinal bias magnetic field caused by a fact that the magnetical thickness x of a magnetic sensitive film decreases by xcex94x by increasing a longitudinal bias magnetic field generated in a magnetic domain control film, it is necessary to make the change xcex94y of the magnetical thickness y of the magnetic domain control film larger than double the change xcex94x of the magnetical thickness of the magnetic sensitive film. Namely, it is necessary to meet xe2x80x9cy greater than =xe2x88x922xxe2x80x9d.
Next, constant xcex1 in expression xe2x80x9cy greater than =xe2x88x922x+xcex1xe2x80x9d is determined in consideration of the minimum magnetic field Hmin of a magnetic sensitive film required for operation. As described above, as a magnetic sensitive film becomes thinner, its anisotropic magnetic field becomes smaller, and finally it reaches the minimum magnetic field Hmin capable of suppressing a Barkhausen noise. The minimum magnetic field Hmin is a design value capable of being obtained by experiences or experiments. Constant xcex1 is given as the minimum magnetic film thickness of a magnetic domain control film capable of applying a longitudinal bias magnetic field exceeding the minimum magnetic field Hmin even in case that the magnetical thickness x of a magnetic sensitive film reaches a minimum value of zero.
According to the present invention, as described above, even in case of making thin a magnetic sensitive film, it is possible to apply a proper longitudinal bias magnetic field to the magnetic sensitive film. Hereupon, the upper limit of the magnetical thickness y of a magnetic domain control film is not set in the above-mentioned expression (1), but when the film thickness of a magnetic domain control film is made excessively large, the longitudinal bias magnetic field is made excessively large and the reproduction sensitivity is degraded, and therefore it is preferable that the value of a magnetical thickness y is set so that such a degradation in reproduction sensitivity does not occur.
In an embodiment of a magnetoresistive element according to the present invention, both ends in the longitudinal direction of a magnetic sensitive portion are made into depressions lower than the middle part of it and a magnetic domain control film is arranged on each of these depressions. In this case, the depth of the depression is set at 5 nm (50 angstroms) or more when taking the surface of the middle part of the magnetic sensitive portion as a reference plane. When the depth of the depression is set at 5 nm or more, a longitudinal bias magnetic field appears clearly.
In case of setting the depth of a depression at 5 nm or more within the film thickness of a magnetic sensitive film, it is preferable to set the magnetical thickness xcex1 (gauss-xcexcm) at 450 (gauss-xcexcm).
In another embodiment of a magnetoresistive element according to the present invention, the depression of a magnetic sensitive portion is formed so that the depth of it exceeds the film thickness of the magnetic sensitive portion. In this case, it is preferable to set the magnetical thickness a (gauss-xcexcm) at 630 (gauss-xcexcm).
In case of an AMR element, its magnetic sensitive film is formed out of an AMR film. In case that a magnetic sensitive portion is of an SV structure, its magnetic sensitive film is formed out of a free layer contained in the SV structure.
A magnetic domain control film may be of a structure comprising an antiferromagnetic film and a magnetic film generating an exchange coupling with the antiferromagnetic film, and may be formed out of a hard magnetic film.
In a magnetoresistive element according to the present invention, it is preferable to set the magnetical thickness x (gauss-xcexcm) of said magnetic sensitive film and the magnetical thickness y (gauss-xcexcm) of said magnetic domain control film so as to almost meet xe2x80x9cy=xe2x88x922x+xcex1xe2x80x9d, but it has been experimentally confirmed that a sufficiently practical magnetoresistive element can be obtained even in case that the value of the minimum magnetical thickness xcex1 of a magnetic domain control film is slipped out by +xe2x88x9210% from the preferable value. Therefore, in a magnetoresistive element according to the present invention, it is possible to set the magnetical thickness x (gauss-xcexcm) of said magnetic sensitive film and the magnetical thickness y (gauss-xcexcm) of said magnetic domain control film so as to meet the following expression:
xe2x88x922x+1.1xcex1 greater than =y greater than =xe2x88x922x+0.9xcex1.xe2x80x83xe2x80x83(2)
And in this case, it is preferable to set the film thickness THx (xcexcm) of said magnetic sensitive film at a value within a range of 5 to 15xc3x9710xe2x88x923 xcexcm.
And the present invention relates also to a method of designing a magnetoresistive element, and in case of designing a magnetoresistive element provided with a magnetic sensitive portion comprising a magnetic sensitive film formed out of a magnetic material responding to a magnetic field applied from the outside and a magnetic domain control film which is formed out of a magnetic material and applies a bias magnetic field in the longitudinal direction in which a sense current flows to said magnetic sensitive film, the invention determines the magnetical thickness x (gauss-xcexcm) of said magnetic sensitive film given by the product of the remnant magnetization Br1 (gauss) of a magnetic material forming said magnetic sensitive film and its film thickness THx (xcexcm), and the magnetical thickness y (gauss-xcexcm) of said magnetic domain control film given by the product of the film thickness THy (xcexcm) of a magnetic material forming said magnetic domain control film and the remnant magnetization Br2 (gauss) of it so as to meet the following expression:
y=xe2x88x922x+xcex1,
or
xe2x88x922x+1.1xcex1 greater than =y greater than =xe2x88x922x+0.9xcex1,
in case of assuming that the minimum magnetical thickness of said magnetic domain control film determined in consideration of the minimum magnetic field Hmin required for operation, of a longitudinal bias magnetic field to be applied to said magnetic sensitive film is a (gauss-xcexcm).
And in case of designing a magnetoresistive element, it is preferable to set the film thickness THx of said magnetic sensitive film at 15xc3x9710xe2x88x923 xcexcm or less, particularly at a value within a range of 5 to 15xc3x9710xe2x88x923 xcexcm and determine the magnetical thickness y (gauss-xcexcm) of the magnetic domain control film so as to meet the above-mentioned conditions.
The present invention relates to a thin film magnetic head in which the above-mentioned magnetoresistive element is formed so as to be supported by a slider, and such a thin film magnetic head brings the above-mentioned action and effects of a magnetoresistive element according to the present invention as they are.